The simple answer is doing everything humanly possible to minimize the delay and disruption, not only to you and your fellow passengers but also (assuming the aircraft is likely to be scheduled for another 5 sectors) the 900+ passengers that are due to use this aircraft later in the day. The only time you are guaranteed handling staff for boarding/loading is prior to your flights scheduled time, and equally passenger behavior dictates that it causes more disruption with the off the aircraft rather than on. If you were offloaded/delayed boarding, the boarding process that usually takes 15-20 minutes is likely to take at least 45 with the two above factors, and then there will be a percentage of passengers that go missing for one reason or the other, and of course they are the ones likely to have a bag in the hold so that's another 15 minutes at least.
By boarding on time you can send a ready message once everyone's on board, definitely, have staff and facilities to do this, and reduce the risk of other delays which may result in you missing a slot (and then further delay to over 1000 people). The pilot saying that there is no chance is done by someone's experience (either his or ops), and also like said before a way of managing expectation, of course, weather can change, and all the sudden the slots disappear...then what? 189 passengers are likely all over the terminal, and it will take at least another 60 minutes to coral everyone back on the aircraft